The invention relates to a method for optically testing the quality of objects which normally have a circular edge, wherein light is directed to the edge of the object. Furthermore, the invention relates to an apparatus for optically testing the quality of such objects, in particular for carrying out the method of the invention, with at least one light-emitting illumination unit for illuminating the edge of each object.
Methods and apparatus of the type under discussion have been known from practical operation for a long time, and they play a more and more important role within the scope of a complete inspection of objects, in particular products of industrial production. These methods and apparatus concern not only a most complete possible inspection of the finished product prior to delivery or a comprehensive acceptance test. Rather, there also exists a developing trend to repeatedly ensure the quality of the intermediate product already between individual manufacturing steps, and to thus enable an early detection of product imperfections or also manufacturing errors.
In the field of the semiconductor industry, a plurality of inspection systems have thus become established over the years, which detect a great variety of criteria in the production of silicon wafers. The efficiency of these systems represents the basis for establishing international standards (SEMI standards, Semiconductor Equipment and Materials International) for the quality of the product “wafer,” so as to make available a worldwide uniform definition for all enterprises operating in the semiconductor industry.
A very critical feature for the quality of wafers is the condition of the wafer surface, where it matters that individual particles, contamination, roughness, and defects be detected with a very high resolution. On their part, the wafer manufacturers intend to perform not only a quality inspection at the end of each production line within the scope of a final inspection, but to provide also such an inspection in several points of the manufacturing process, and to ensure that faulty silicon slices are sorted out at an earliest possible time as a function of the type or the distribution of imperfections, for purposes of thus preventing further processing in the production which is connected with high costs, or even a delivery of such slices to customers.
Visually detectable defects, even when these are not obvious and visible only with special illumination and with the aid of magnifying optical systems, lead at the customers' end largely to the following problems:
1. Early detection of defects (for example, within the scope of the acceptance test) normally leads to a complaint and return of the delivered wafer. A consequence resulting therefrom is, for example, the delay in further processing and, thus, a corresponding outage of production times.
2. If defects are detected only in the further manufacturing process or at the end of further processing, it will become necessary to separate a costly manufactured and thus a higher valued product as a whole. This unnecessarily reduces the actual production capacity on the one hand, while on the other hand, costs for the rejected product have already been incurred within the scope of further processing.
3. If an undetected defect even leads to a break of the wafer in the course of further processing, one will have to add to the mere material costs, which in this case already amount per se to considerable sums, the costly restoration of clean-room conditions after such an occurrence and the therewith connected shutdown of an entire production line.
In the current state of the art, the automated inspection of the wafer surface with respect to particles, roughness, and defects excludes the edge zone of the wafer from the inspection. This edge zone is defined by the SEMI standards as the transitional region from the front side of the wafer to the backside thereof, and moreover respectively a range of three millimeters starting from the edge region into the surface of the wafer.
Since the edge region of the wafer (=edge+part of the edge zone, in particular the exclusion zones as are defined by current SEMI standards in the inspection of surfaces) is at the present totally excluded from the definition of quality standards for wafer surfaces, the edge region of the wafer is currently subjected only to a manual, visual inspection by an operator. In this inspection, very strong light sources are used as auxiliary devices to be able to detect possible defects on the edge of the wafer. In this process, one examines in particular the edge for light reflections, which are caused by unevennesses. However, from the viewpoint of manufacture and further processing, the reliability and reproducibility of this visual method are the worst conceivable, which has until now prevented defining quality standards in an analogous manner to the front and the backside.
Besides the visual, scattered-light inspection by an operator, which must be considered inherently subject to errors, there currently exists only one automated device for inspecting wafers, namely the “edge scan” device from Raytex, which examines the edge of the wafer, but not the edge zone. This system that is based on a scattered-light evaluation of the laser beam which is perpendicularly directed to the edge, but this system inadequately meets customer demands. On the one hand, this is due to a too low resolution of the system (one measured value for 25 μm of the wafer circumference) and the resultant limited sensitivity. Furthermore, the scattered-light evaluation as used is sensitive to deviations of the wafer from its ideal geometry (warping), the profile of the edge that is to be made, and the handling of the wafer during the measurement itself (tilting of the wafer relative to the axis of the laser beam).
A classification of defects by their more specific type (contamination, scratches, chipping, coating, particles, etc.) within the scope of the visual inspection has until now been possible only in a further, time-consuming step by examining the damaged wafer in greater detail under a microscope. For the available automated system, an assessment of detected imperfections is likewise ensured only by an operator, who must analyze to this end a camera picture of each defect. With that, more extensive, statistic tests and detection of systematic errors have until now likewise not been possible or realized.
As a whole, the edge of a wafer becomes more and more important, which results from the edge handling of wafers with a diameter of 300 mm as is recommended by the SEMI standard. This region is thus exposed to additional, mechanical stresses, which in part only cause defects that are to be detected, or which sensitively react to existing defects.
Since the demand for larger wafer diameters steadily increases, and quality requirements become greater to the same extent, there exists, in particular on the part of the wafer producers, a need for a fully automated system for inspecting the wafer edges with respect to defects and roughness, namely a system which is in a position to measure silicon slices accurately, to standards, in a manner free of contamination and destruction in and between the corresponding production steps of the wafer manufacture, and to detect and classify imperfections.
It is therefore an object of the present invention to improve and further develop a method and an apparatus of the initially described type for optically testing the quality of objects with a circular edge in such a manner that they permit a reliable, reproducible inspection of the object edge with great accuracy.